This invention generally relates to solid immersion lenses and collimators using such solid immersion lenses.
In many optical systems and applications, such as near-field microscopy, imaging, photolithography and optical storage it is important to reduce the spot size and thus obtain higher definition or resolution. The spot size of an optical system, e.g., an optical storage system, is commonly defined as the distance between half power points. This distance is determined by diffraction to be approximately xcex/(2xc2x7NA), where xcex is the free space wavelength of the light used and NA is the numerical aperture of the objective lens focusing the light beam. NA is defined as NA=nsin(xcex8), where xcex8 is the half cone angle of the focused light rays and n is the index of refraction of the medium in which xcex8 is measured.
One way to improve the definition is to work at shorter wavelengths xcex, e.g., in the green or blue range, and to increase the numerical aperture to be as close to one as possible. A further possibility is to employ near-field optics in the manner described by Betzig et al. in Applied Physics Letters, Vol. 62, pp. 142 (1992), using a tapered fiber with a metal film with a small pinhole at the end. The definition of the system is determined by the size of the pinhole, and can be 50 nm or less. The advantages of the fiber probe system are its excellent definition and its polarization preserving capability which is particularly useful in magneto-optic storage applications. The disadvantages of the system are its poor light efficiency and the fact that it can only observe a single spot at a time, thus limiting its tracking ability when used for optical storage.
Another alternative is to use a solid immersion lens (SIL) between the objective lens and the illuminated object, e.g., an optical recording medium or sample under investigation. The SIL is placed within a wavelength xcex or less (in the near-field) of the object. Optical systems taking advantage of appropriate SILs are described, e.g., by S. M. Mansfield et al. xe2x80x9cSolid Immersion Microscopexe2x80x9d, Applied Physics Letters, Vol. 57, pp. 2615-6 (1990); S. M. Mansfield et al. xe2x80x9cHigh Numerical Aperture Lens System for Optical Storagexe2x80x9d, Optics Letters, Vol. 18, pp. 305-7 (1993) and in U.S. Pat. No. 5,004,307 issued to G. S. Kino et al. In this patent Kino et al. teach the use of a high refractive index SIL having a spherical surface facing the objective lens and a flat front surface facing an object to be examined. The use of this SIL enables one to go beyond the Rayleigh diffraction limit in air. In one embodiment, the SIL is employed in a near-field application in a reflection optical microscope to increase the resolution of the microscope by the factor of 1/n, where n is the index of refraction of the SIL.
A paper by G. S. Kino presented at the SPIE Conference on Far- and Near-Field Optics, xe2x80x9cFields Associated with the Solid Immersion Lensxe2x80x9d, SPIE, Vol. 3467, pp. 128-37 (1998) describes in more detail the principles of operation of two particular SILs. The first is a hemispherical SIL and the second is a supersphere SIL or a stigmatic SIL. The hemispherical SIL improves the effective NA of the objective lens by the refractive index n of the SIL and decreases the spot size by 1/n. The supersphere SIL increases the effective NA of the objective lens by the square of the refractive index n2 and obtains a focus at a distance a/n from the center of the supersphere, where a is the sphere""s radius. The spot size is reduced by a factor of n2. The performance characteristics and theoretical limitations of both types of SILs are also discussed.
SILs have found multiple applications. For example, Corle et al. in U.S. Pat. No. 5,125,750 teach the use of a SIL in an optical recording system to reduce the spot size in an optical recording medium. These SILs typically have a spherical surface facing the objective lens and a flat surface facing an optical recording medium. The flat surface is in close proximity to the medium.
In U.S. Pat. No. 5,497,359 Mamin et al. teach the use of a superhemisphere SIL in a radiation-transparent air bearing slider employed in an optical disk data storage system. Lee et al. in U.S. Pat. No. 5,729,393 also teach an optical storage system utilizing a flying head using a SIL with a raised central surface facing the medium. In U.S. Pat. No. 5,881,042 Knight teaches a flying head with a SIL partially mounted on a slider in an optical recording system. This slider incorporates the objective lens and it can be used in a magneto-optic storage system. Finally, in U.S. Pat. No. 5,883,872 Kino teaches the use of a SIL with a mask having a slit for further reducing the spot size and thus increasing the optical recording density in an optical storage system, e.g., a magneto-optic storage system.
The prior art SILs as well as the optical systems using them have a number of shortcomings. Hemispherical SILs suffer from back reflection problems. These degrade system performance, especially when the light source is a laser, e.g., a laser diode, and the back reflection is coupled back into the laser. Also, the ray reflected from the spherical surface and the ray reflected from the flat surface or from an object just below the flat surface are coincident. This gives rise to undesirable interference effects.
Superhemispherical SILs have reduced back reflection. However, they demagnify the image of the object by a larger factor than hemispherical SILs. For example, the demagnification of superhemispherical SILs in the axial direction is 1/n3. Because of this, the length tolerance for the superhemispherical SIL is very tight. Both the hemispherical and superhemispherical SILs increase the effective NA (NAeff) of the objective lens (for hemispherical SIL NAeff=NAobjectivexc2x7n; and for superhemispherical SIL NAeff=NAobjectivexc2x7n2). The maximum NAeff that can be obtained by either type of SIL is NAeff=n.
Hemispherical, superhemispherical and related SILs experience alignment problems because optical systems employing them require the use of a separate objective lens. This separate lens has to be accurately aligned with the SIL. In many optical systems alignment between these two lenses cannot be easily preserved due to external influences (vibrations, stresses, thermal effects etc.). In addition, in systems where the number of parts is to be small, e.g., for weight and size reasons the objective lens is cumbersome.
Also it is well known in the art that a plurality of lenses (or micro-lenses) can be fabricated within a monolithic body in an ordered arrangement to comprise a lens array (or micro-lens array). Such lens arrays may be formed by processes that include photolithography, etching, ion milling, reflowed photoresist methods, molding, and thermal bonding methods as described in Optics and Photonics News, Sep. 1999, pp. 19-22, and in xe2x80x9cMicroopticsxe2x80x9d, by Stefan Sinzinger and Jurgen Jaohns, Wiley-VCH, 1999. Unfortunately, the arrays disclosed in the prior art do not effectively overcome the difficulties associated with attaching multiple waveguides to a monolithic body in precise alignment with the corresponding lenses.
In addition, there is a need in the industry to develop effective, light-weight and easy to use collimators for waveguides such as optical fibers. The fusing of lenses, e.g., graded index lenses (GRINs), to the ends of fibers is known and described, e.g., in U.S. Pat. No. 4,737,006 to Warbrick, U.S. Pat. No. 4,962,988 to Swann and U.S. Pat. No. 6,033,515 to Walters et al. These patents also teach techniques for performing fusion splicing of a lens to the fiber. Additional fusion splicing techniques are described, e.g., in U.S. Pat. No. 5,299,274 to Wysocki et al. and U.S. Pat. No. 5,745,311 to Fukuoka et al. These and other prior art fusion spliced parts and splicers attempt to overcome alignment problems encountered in these techniques.
Unfortunately, prior art SILs require additional objective lenses, as mentioned above, and require precise alignment with those. Hence proper splicing with a waveguide, e.g., a fiber, is only one of the problems. Monolithic arrays further increase the difficulties associated with proper alignment as multiple waveguides must now be precisely aligned with each SIL. It would be an advance in the art if SILs and SIL arrays which are less tolerant to alignment problems could be developed for fusion splicing with waveguides and advantageously used to collimate the respective diverging beams of light emerging from the waveguides. It would also be an advance in the art to develop micro-SIL arrays for respectively collimating the diffraction-limited Gaussian beams emerging from a large number of single-mode optical fibers arranged in a high density packaging configuration (fiber array).
Accordingly, it is a primary object of the present invention to provide a solid immersion lens (SIL) which overcomes the prior art limitations and ensures a small spot size. It is a specific object of the invention to integrate the objective lens and the solid immersion lens into a single collimator.
It is a further object of the invention to provide such an integrated SIL for fusion splicing applications with waveguides such as optical fibers. Additionally, it is a specific object of the invention to provide means for reinforcing the attachment of waveguides to the SILs.
It is another primary object of the invention to provide a monolithic array which overcomes the prior art shortcomings in precisely aligning attached waveguides.
Further objects and advantages will become apparent upon reading the detailed description.
The objects and advantages of the invention are secured by a collimator integrated with a waveguide and employing an ellipsoidal solid immersion lens (ESIL). The ESIL has a substantially uniform index of refraction n, an ellipsoidal surface portion defining a geometrical ellipsoid with a major axis M, a first geometrical focus F1 and a second geometrical focus F2 separated from first geometrical focus F1 by a separation S=M/n. The collimator has an attachment surface portion passing substantially through second geometrical focus F2. The attachment surface portion is for attaching the ESIL to the waveguide such that a collimated light beam propagating along major axis M through the ellipsoidal surface portion converges to a focus substantially at second geometrical focus F2 or at the waveguide.
The attachment of the attachment surface to the waveguide can be performed in many ways. The manner in which the waveguide and ESIL are joined can be adapted to the type of waveguide, e.g., an optical fiber or a buried waveguide. In one embodiment the attachment surface is attached to the waveguide by a fused butt joint.
In one convenient embodiment of the invention the ESIL has a body and a pedestal. The body has the ellipsoidal surface portion through which light passes. The pedestal has the attachment surface portion by which the ESIL is attached to the waveguide. The pedestal can have a pedestal cross section dimensioned to match the waveguide. For example, when the waveguide is an optical fiber the pedestal cross section can equal that of the optical fiber. The pedestal cross section can also be tapered, e.g., it can be tapered down from a larger cross section to the cross section of the waveguide at the attachment surface portion. In another embodiment the ESIL has a cross section matched to the waveguide.
The ESIL, or more generally any SIL, can further be integrated into a monolithic body. The substrate of this monolithic body allows for multiple SILs to be combined in one body. A monolithic body incorporating an array of SILs can be formed using photolithography, etching, ion milling, reflowed photoresist methods, molding, or other common processes. A useful embodiment of the invention has pedestals on the attachment surface portion of the substrate to provide for low-loss coupling and increased precision in attaching waveguides, resulting in improved pointing accuracy of the collimator. A reinforcing structure may also be employed to stabilize the attachment of waveguides to the monolithic body.
The ESIL can be made of one or more sections, depending on the application of the collimator and design requirements. However, it is preferable that the attachment surface portion be flat for easier attachment, e.g., by fusion bonding to the waveguide.